Voltages used in power transmission systems are getting higher and higher. Voltage levels in the range of 600-1200 kV are known to be used.
At these levels the size of the equipment used is large, which is especially the case with inductive devices such as transformers. A transformer that is to operate at the above-mentioned voltage levels may be so large and bulky that it is hard to transport. The different components, such as the windings may then also become heavy.
There is therefore a need for limiting the size. At the same time it is important that the insulation is able to provide sufficient insulation. Insulation is usually provided through the use of cellulose and transformer oil.
Although the above mentioned issues are important at high voltage levels, it should be realized that they may also be of interest at lower voltage levels.
One way to reduce the size of inductive devices is disclosed in JPS 62-126609. JPS 62-126609 discloses a foil wound transformer, where low voltage windings are provided around high voltage windings. Furthermore high voltage lead wires lead out from the inner high voltage windings in the center between the outer windings.
In a transformer of the above-described type it is also necessary to provide a shielding of the winding through which the lead wires lead. The traditional way of providing such shielding is through providing the shielding as two separate thin shielding rings at the top and bottom of the winding. However, then the electrical field will be non-uniform and high at the shielding rings.
It is also necessary to provide insulation between the central wires and the windings through which they pass. This insulation may be hard to design.
One problem that faces the design of the insulation is that the above-mentioned shielding system cause high dielectric creep stresses in the insulation.
Another problem that may face the design of the insulation is how to design it so that it is also able to receive the assembly forces as well as the short circuit forces on the outer winding.
There is in this respect a need for an improved shielding that solves at least some of the problems mentioned above.